Preparation of metals and alloys



2220.110 .PZUQ mum 2 Sheets-Sheet l W. A. KRIVSKY PREPARATION OF METALS AND ALLOYS Filed Nov. 18. 1960 May 24, 1966 INVENTOR. WILLIAM A. KRIVSKY NOEIUVD mas 83d kw Q Haw ATTORNEY May 24, 1966 w. A. KRIVSKY PREPARATION OF METALS AND ALLOYS 2 Sheets-Sheet.- 2

Filed Nov. 18, 1960 w w vn I n Xf r O wii I Ill N.m|.m /h q e M II, m X .I. I WW 0A A 6 O m M m m m m. 20220 .560 mum 4a 52 PER CENT CHROMIUM n B as V. X o 9 rl U P Oxygen-Argon Mixture Started Here wmmu OxygenzArgon Equilibrium Relationship i=1 WILLIAM A. KRIVSKY 24 PER CENT CHROMIUM INVENTOR.

BY M Q. llmw.

A TTORNE) United States Patent Office 3,252,7hd Patented May 24:, 1966 3,252,790 PREPARATEUN 01F METALS AND ALLOYS William A. Krivslry, Fremont, Ohio, assignor to Union Carbide Corporation, a corporation of New York Filed Nov. 18, 1960, Ser. No. 71,378 4 Claims. (Cl. 75-60) This application is a continuation-in-part of US. application Serial No. 741,511 filed June 12, 1958, now abandoned, which in turn is a continuation-in-part of U.S. application Serial No. 594,287, filed June 27, 1956, now abandoned.

This invention relates to a method for purifying metals and alloys by removing non-metallic, volatilizable impurities therefrom.

It is conventional practice to remove excessive quantities of certain impurities present in metals and alloys by introducing reactive gases into a molten mass thereof. The limiting factor determining the eflicacy of this practice is the equilibrium existing between a given impurity, the gas, and one or more constituents of the melt. One example of such practice is afforded by the process called decarburization, which involves injecting oxygen into molten steels to react with the carbon dissolved therein to form removable volatile carbon oxides. In the case of carbon in chromium-containing ferro-alloys, an established equilibrium exists between carbon, iron and chromium. During normal pure oxygen injection, all the carbon in excess of that in equilibrium with the given amount of chromium at a given temperature is oxidized and evolved as a carbon oxide.

, Carbon-chromium equilibria have been experimentally determined, and data are available showing that the final carbon content generally decreases with decreasing chromium content, increasing temperature and decreased partial pressure of the evolved gas. Attempts to lower the carbon content below the equilibrium value without altering the above variables results in the oxidation of chromium as well as that of carbon.

Accordingly, the problem confronting workers in this art is to reduce the impurity content below its equilibrium value at atmospheric pressure respective to other melt constituents without affecting any one of these in an unwarranted manner.

Currently two expedients partly resolve the above problem. When it is desired to produce high alloy steels with low carbon content, the temperature of the melt may be increased to achieve a more favorable equilibrium to remove additional carbon by normal oxygen injection, in which case a cost penalty results in the form of rapid refractory deterioration. A limit to the extent to which the temperature may be elevated is also encountered, which fixes the lowest carbon content obtainable with any given alloy composition. An alternative expedient in the attainment of a high degree of decarburization is vacuum melting. The additional expense involved by this technique is well known.

It is an object of this invention to provide a method for removing impurities from metals and alloys by reacting the impurities with a reactant gas whereby the impurities are converted to volatile compounds which method permits impurity removal beyond the equilibrium level at atmospheric pressure.

It is another object of the present invention to provide a process for the oxidation of carbon impurities in metals and alloys to effect its removal whereby the carbon may be removed beyond the equilibrium level at atmospheric pressure without necessitating excessively high temperatures or vacuum melting techniques.

Still another object of the present invention is to provide a process for oxidizing carbon to effect its removal from carbon-containing chromium-iron alloys beyond the equilibrium value at atmospheric pressure which process minimizes chromium losses by oxidation without necessitating excessively high temperature or vacuum melting techniques.

Another object of the present invention is to provide a process for removing more carbon from ferro-alloys containing chromium than is presently accomplished at known carbon-chromium-oxygen equilibria at sub-atmospheric conditions without necessitating excessively high temperature.

Other objects and advantages of this invention will become apparent as the description thereof proceeds, especially when viewed in connection with the accompanying graphical data in which:

FIG. 1 illustrates curves representing a conventional carbon-chromium equilibrium, and a curve showing that obtainable with the invention;

FIG. 2 presents an extension of the pure oxygen curve for 1600 C. as shown in FIGURE 1 for carbon and chromium at high chromium levels; and

FIG. 3 shows the curves of FIG. 1 for the pure oxygen equilibrium of the prior art in comparison with the 1.1 argon to oxygen relationship of the present invention.

The process which satisfies the objects of the present invention comprises introducing a reactive gas into a molten impurity containing alloy to react with the impurity and convert the same to a gaseous compound and adding an inert gas at least during the period when the impurity content of the molten alloy is below the level normally existing at equilibrium at atmospheric pressure, whereby the partial pressure of the evolved gaseous compound is lowered.

Another manner of stating the point at which the inert gas is injected which describes substantially the same point as recited above is to state that the inert gas is injected at least from the time that the reactive gas begins to substantially oxidize and cause loss of the alloy constituents of the molten alloy being treated.

This process is useful when the primary source of heat for maintaining the temperature of the molten bath is the exothermic reaction of the reactive gas with the impurities. In this case it is essential that the rate of reactive gas input be maintained at least approximately constant to that amount which would be used when no argon addition is employed. Preferably, the input rate should be increased slightly, up to as much as about 10 percent for maximum efficiency. The inert gas may. be added prior to the point at which the carbon reaches the atmospheric pressure equilibrium value. However, the primary advantage is obtained if the inert gas is added during the period when the carbon content approaches the atmospheric pressure equilibrium value and throughout the period when the carbon content is below the equilibrium value.

In all cases during the present process the temperature of the molten bath is sustained to maintain the oxidizing power of the reactive gas in relation to the impurity and to prevent a substantial increase in the oxidizing power of the reactive gas in relation to the alloy constituents of the molten alloy.

Some variance in temperature may be permitted during the process but substantial decreases in temperature during the process or rapid cooling of the melt will nullify the beneficial effects of the introduction of inert gas because 'the effect of lowering the partial pressure of the evolved gas in removing the impurity will be lost if the temperature of the melt is allowed to fall to a temperature where the oxidizing power of the reactive gas in relation to the alloy constituents exceed the oxidizing power of the reactive gas in relation to the impurity. It should be noted that as temperature increases metal is more stable than carbon in the presence of oxygen.

The temperature at the end of the reaction conducted in the present process also must not be substantially' lower than the temperature during the reaction between impurity and reactive gas.

The allowable temperature variance during the process is dictated by known equilibria at various temperatures for a given reactive gas and impurity. The allowable temperature variance will be illustrated in relation to ferro-alloys containing carbon as an impurity and chromium as an alloying agent with oxygen as the reactive gas. In all cases where the present process is utilized, a temperature increase during the reaction is preferred if the refractory lining of the reaction vessel will so permit although a temperature increase is not required to obtain the desired results of the present process, namely, removal of impurities without substantial loss of alloy constituents for a given temperature of the melt and pressure surrounding the melt. The limit of temperature increase is defined by the refractory lining temperature limit and the volatility of alloy constituents at elevated temperatures.

The impurity-alloy constituent-oxygen relationship at a given temperature is conveniently illustrated in reference to ferro-alloys containing chromium with carbon as the impurity. The common practice utilized to remove carbon involves introduction of pure oxygen into the ferro-alloy. The final carbon content obtainable is predicted from a set of curves put forth by Chipman, for example, in a article in the Journal of the Iron and Steel Institute, vol. 180, p. 97-106, entitled, Atomic Interaction in Molten Alloy Steel, and also by Hilty, Rassbach, and Crafts in The Journal of the Iron and Steel Institute entitled, Observations of Stainless Steel Melting Practices," vol. 180, p. 116-128. These relationships are widely used in industry. The pure oxygen curve shown in FIGURE 1 illustrates a plot of the equilibrium for given carbon and chromium contents in a ferro-alloy utilizing pure oxygen introduction at a temperature of 1600 C. as shown by the above authors. In the general form, the relationship for the prior art equilibrium between chromium, carbon, oxygen and temperatures in a ferro-alloy is shown by the following equation:

where M O is chromite, an iron-chromium spinel, Cr O or an oxide of manganese depending upon the amount and type of alloy constituent in the molten ferro-alloy as shown by Hilty et al. and Chipman.

The equilibrium constant for this reaction can be computed, for example, from Chipmans activity coeflicients at various temperatures for various alloy constituents. The carbon content obtainable for each set of conditions in a ferro-alloy containing chromium as the alloy constituent is expressed by the following relationship where pure oxygen is utilized to remove carbon and the pressure surrounding the melt is at approximately one atmosphere.

Percent (3:?) (Percent Cr) K,

where K, is the equilibrium constant derived-from the activities of carbon and chromium at a given temperature as put forth by Chipman.

Accordingly a family of curves similar to the pure oxygen curve of FIGURE 1 may be plotted for each temperature from the above relationship. In all cases, for a given carbon level, the above relationship dictates that temperature must increase to obtain an increase in chromium content of the finished product for lower carbon levels. The only way to maintain a lower carbon content at a given chromium or a higher chromium content at a given carbon content, therefore, is to increase the temperature of the molten bath during the carbon-oxygen reaction. As higher chromium contents are desired at lower carbon contents the temperautre requirements exceed the limits of the refractory linings in the reaction vessels. The standard practice has been to reduce the carbon content to as low as possible at a permitted operating temperature by introduction of pure oxygen and then ceasing the introduction of oxygen and adding a reducing agent to cause reduction of the oxidized chromium from the slag to cause the alloying element chromium to pass back to the molten metallic melt.

The above relationship for chromium-carbon can be modified to cover pressures lower than 1 atmosphere by multiplying the right side of the relationship by the pressure in atmospheres surrounding the melt itself. The present method contemplates adding the reactant gas, for example, oxygen, with an inert gas such as argon. The resultant reaction product gas, carbon monoxide, containing argon, now having a lower partial pressure of carbon monoxide than when oxygen is injected alone, permits a lower carbon content to be obtained for any given chromium content and temperature level and therefore the use of reducing agents to recover chromium from the slag is not required.

The present process is also amenable for use under vacuum melting conditions. In this case the above general formula for ferro-alloy containing chromium will be modified to the following as stated above.

Percent C= 4 (Percent where P is the pressure surrounding the melt itself. The use of inert gas in accordance with the present process will again allow an artisan to obtain a lower carbon content than shown by the above relationshipfor a given percent Cr, K temperature and pressure.

In the present process it is readily seen that the temperature of the molten fero-alloy may vary somewhat but must be kept within certain limits. That is the beneficial etfects of the introduction of inert gas may be completely lost if the melt is not maintained at least above about 1550 centigrade when chromium is the alloying element. The upper limit of temperature is dictated only by the operable temperature of the refractory lining in the reaction vessel and in chromium containing melts a temperature increase during the present process is preferred but not required to obtain a final ferro-alloy having a lower carbon content for a given chromium and temperature level than has been previously possible to produce at any given pressure surrounding the melt itself. In alloys containing up to about 40 weight percent chromium, the upper limit of the temperature range may be about 2500 degrees centigrade.

By way of example, the results of several heats are shown on the appended graphs, wherein the beneficial action of blowing iron-chromium alloys with oxygen-argon mixtures is shown. In all of the following examples the oxygen input was maintained and the inert gas was added thereto.

Referring to FIG. 1, the upper curve (pure oxygen curve) represents the carbon-chromium equilibrium where only pure oxygen is used at 1600 C. as shown in the prior art. Here the lowest carbon content obtainable in ironchromium alloys, where chromium ranges from 0 to 36 percent, is shown. The lower curve in the same figure illustrates the marked decrease in carbon content obtainable when a 1 to 1 mixture of oxygen to argon is used. This curve was established by a series of tests wherein the gas mixture was blown into pound heats, which were melted in an induction furnace, and held at 1600 C. In this figure the tails of the arrows represent the initial melt compositions before oxygen-argon injection, and the heads of the arrows indicate the final composition after refining. These illustrate the marked drop in carbon content possible by the practice of the invention. Heat marked A in the curve was run with pure oxygen. It can be seen that the carbon content fell to the equilibrium content here, and reduced further only in proportion to the oxidation of the chromium in the melt. Other heats marked D and E in which the original chromium content ranged between 14.2 percent to 19.2 percent, and between about 4 percent to 6 percent, respectively, were treated with an oxygen-argon mixture having a ratio of 1 to 1. Heats B and C show the effect of heavy argon concentration in the gas mixture, and further show that decarburization was effected considerably below the level accomplished with the normal 1 to 1 oxygen to argon mixture. In all cases where no break appears in the arrow, maximum decarburization was not obtained, but could have been reached by further blowing with the gas mixture without substantial chromium loss. FIG. 2 illustrates the normal equilibrium relationship between carbon and chromium at high chromium levels at 1600 C. In this instance oxygen refining with pure oxygen only increased the carbon content by the oxidation of chromium alone, whereas blowing a gas mixture comprising oxygen and argon in approximately a 1 to 4 ratio lowered the carbon content materially.

In the practice of the present invention, it is not necessary to use a reactive gas-inert gas mixture throughout the entire impurity removal process. In some processes it may be desirable to use only the reactive gas during the early stages of the operation, and add the inert gas or inert gas mixture during the finishing stages of the operation. The manner of proceeding is determined by the particular material treated and the economics of supplemental gas consumption and heat utilization. In this connection, FIG. 3 illustrates the practicability of modifying the method of the invention by employing supplemental gas only at the end of the treating process. In this case the standard oxygen blowing practice for iron-chromium alloys was followed by the introduction of a 1 to 1 oxygen-argon gas mixture. The graphs show a drop in carbon content to the equilibrium level, using pure oxygen and the subsequent oxidation of chromium only where pure oxygen was employed. Additional decarburization is also shown without the attendant loss of chromium, where the oxygen-argon mixture was used. The particular reactive gas to inert gas ratio which may be used is determined mainly by economical considerations. Generally, the higher the ratio, the more beneficial the results. All things being equal, an inert gas-reactive gas ratio range of between about 1:1 to 8:1 is practical for most purposes.

The quantity, pressure and velocity of the gas used may conform to that employed in standard top blowing procedures for example. Such factors are Well known to those skilled in the art.

While argon has been mentioned throughout the above specific examples relating to the decarburization of ironchromium alloys, other inert gases such as helium, neon, krypton, xenon, nitrogen and hydrogen, where applicable, may be employed.

Although the invention has been illustrated by specific examples relating to the decarburization of iron-chromium alloys, its applicability is not limited thereto. The

method of the invention is applicable to other processes where it is sought to remove an impurity capable of being evolved directly as a gas or as a part of a gaseous compound. The method of the. invention is applicable also wherever it is possible to reduce the partial pressure of an evolved impurity-containing gas by supplementing the reactive gas used to evolve this impurity by means of gas inert thereto.

It has been stressed in this disclosure that the reactive gas input rate during the practice of this invention be maintained at substantially the same rate as when the reactive gas is used alone as in regular practice when the heat supplied to the melt is derived primarily from the reactive gas-impurity reaction. To illustrate this, a series of tests were run wherein the oxygen decarburization was followed by a dry air blow. The oxygen rate was 30,000 cubic feet per hour whereas the dry air blow rate was 13,000 cubic feet per hour. As may be seen not only was the total volume of gas in the air blow less than for 6 the oxygen blow, but the oxygen content was still further decreased by dilution withthe atmospheric nitrogen. These tests were performed on two 25-ton heats of Type 304 stainless steel. Results of these tests are shown in Table I.

Table I Heat 1 Heat 2 Carbon content at end of O2 blow, percent 0. 009 0. 033 Carbon content at end of air blow, percent 0. 082 0. 040 Pro-tap carbon content, percent 0.080 0. 053

Table II 1:1 02-N2 Blow Heat 1 Heat 2 Carbon content before blow, percent 0. 41 0. 41 Carbon content after blow, percent 0.25 0. 26

In each of the examples of Table II the equilibrium carbon content was calculated to be in the range of 0.40 percent to 0.45 percent. In each of the examples of Table II the final chromium content was greater than predicted by pure oxygen equilibria.

In all of the preceding examples a standard type gas injection lance was employed. A refractory coated standard steel has been found to be suitable for injecting the oxygen and inert gas into the molten metal.

In a series of tests molten steels containing from about 0.3 to about 0.5 percent carbon were blow with oxygen until the carbon content was reduced to about 0.1 percent. At this point the oxygen input was continued constant and additional argon was blown into the molten metal such that the ratio of oxygen to orgon was 1 to 1. By this procedure the carbon content was reduced to about 0.016 percent while retaining a higher chromium content than possible with pure oxygen introduction. Similar tests under substantially the same operating conditions employing only oxygen as the blowing agent produced steels containing about 0.024 percent carbon or higher with high chromium losses.

While it has been found that an inert gas-reactive gas ratio range of between about 8 to 1 to about 1 to 1 is practical for most purposes, the invention may also be utilized by continuously increasing the inert gas-reactive gas ratio from a value of zero up to greater than the 8 to 1 during the progress of the impurity removal.

What is claimed is:

1. A process for removing carbon from molten ferroalloys containing about 4 to 40 percent chromium as an alloying element and containing carbon comprising adjusting the temperature of said molten ferro-alloy to a range between 1550 to 2500 degrees centigrade, first introducing gaseous oxygen into said molten ferro-alloy to cause at least a portion of said oxygen to react with said carbon to evolve carbon oxide and continuing said introduction of oxygen until said oxygen begins to oxidize and cause loss of the chromium from said molten ferroalloy then introducing at least one inert gas selected from the group consisting of argon, xenon, neon nitrogen and helium into said molten ferro-alloy while continuing said oxygen introduction to cause a reduction of the. partial pressure of said carbon oxide evolved in said molten ferro-alloy to a pressure lower than the pressure surrounding said molten ferro-alloy and simultaneously sustaining the temperature of said molten ferro-alloy within said adjusted range, without decrease to maintain the oxidizing power of said oxygen in relation to said carbon and to prevent an increase in the oxidizing power of said oxygen in relation to said chromium, whereby carbon is removed from said molten ferro-alloy to a value less than that value of carbon dictated by the following relationship with only minor loss of said chromium in said molten ferro-alloy Percent C= (Percent Cr) (P) where K, is the equilibrium constant for the chromium oxide-carbon reaction at temperature T, said temperature T being the temperature of said molten ferro-alloy during said process and being within said adjusted range and where percent Cr is approximately the value of said chromium present in said ferro-alloy during said process and where P is the pressure surrounding said molten ferro-alloy during said process.

2. A process for removing carbon from molten stainless steels containing up to 30 percent chromium as an alloying element comprising adjusting the temperatureof said molten stainless steel to a range between 1550 and 2000 degrees centrigrade, introducing oxygen and at least one inert gas selected from the group consisting of argon, xenon, neon, nitrogen and helium into said molten stainless steel to cause at least a portion of said oxygen to react with said carbon to evolve carbon oxide and to cause a reduction of the partial pressure of said carbon oxide evolved in said molten stainless steel to a pressure lower than the pressure surrounding said molten stainless steel and simultaneously sustaining the temperature of said molten stainless within said adjusted range, without decrease to maintain the oxidizing power of said oxygen in relation to said carbon and to prevent an increase in the oxidizing power of said oxygen in relation to said chromium, whereby carbon is removed from said molten stainless steel to a value less than that value dictated by the following relationship and with only minor loss of said chromium in said molten ferro-alloy Percent C= (Percent C10 (P) References Cited by the Examiner UNITED STATES PATENTS 1,034,785 8/191-2 Greene -59 X 1,034,786 8/1912 Greene 7559 X 1,034,787 8/1912 Greene 7559 X 1,792,967 2/ 1931 Clark 7559 FOREIGN PATENTS 472,397 9/1937 Great Britain.

BENJAMIN HENKIN, Primary Examiner. RAY K. WINDHAM, MARCUS U. LYONS, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,252 ,790 May 24 1966 William A. Krivsky It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 25, Column 7, line ll and Column 8, line 9, the equation, each occurrence, should appear as shown below:

Column 6, line 41, "blow" should read blown line 45, "orgon" should read argon Signed and sealed this 16th day of September 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer 

1. A PROCESS FOR REMOVING CARBON FROM MOLTEN FERROALLOYS CONTAINING ABOUT 4 TO 40 PERCENT CHROMIUM AS AN ALLOYING ELEMENT AND CONTAINING CARBON COMPRISING ADJUSTING THE TEMPERATURE OF SAID MOLTEN FERRO-ALLOY TO A RANGE BETWEEN 1550 TO 2500 DEGREES CENTIGRADE, FIRST INTRODUCING GASEOUS OXYGEN INTO SAID MOLTEN FERRO-ALLOY TO CAUSE AT LEAST A PORTION OF SAID OXYGEN TO REACT WITH SAID CARBON TO EVOLVE CARBON OXIDE AND CONTINUING SAID INTRODUCTION OF OXYGEN UNTIL SAID OXYGEN BEGINS TO OXIDIZE AND CAUSE LOSS OF THE CHROMIUM FROM SAID MOLTEN FERROALLOY THEN INTRODUCING AT LEAST ONE INERT GAS SELECTED FROM THE GROUP CONSISTING OF ARGON, XENON, NEON NITROGEN AND HELIUM INTO SAID MOLTEN FERRO-ALLOY WHILE CONTINUING SAID OXYGEN INTRODUCTION TO CAUSE A REDUCTION OF THE PARTIAL PRESSURE OF SAID CARBON OXIDE EVOLVED IN SAID MOLTEN FERRO-ALLOY TO A PRESSURE LOWER THAN THE PRESSURE SURROUNDING SAID MOLTEN FERRO-ALLOY AND SIMULTANEOUSLY SUSTAINING THE TEMPERATURE OF SAID MOLTEN FERRO-ALLOY WITHIN SAID ADJUSTED RANGE, WITHOUT DECREASE TO MAINTAIN THE OXIDIZING POWER OF SAID OXYGEN IN RELATION TO SAID CARBON AND TO PREVENT AN INCREASE IN THE OXIDIZING POWER OF SAID OXYGEN IN RELATION TO SAID CHROMIUM, WHEREBY CARBON IS REMOVED FROM SAID MOLTEN FERRO-ALLOY TO A VALUE LESS THAN THAT VALUE OF CARBON DICTATED BY THE FOLLOWING RELATIONSHIP WITH ONLY MINOR LOSS OF SAID CHROMIUM IN SAID MOLTEN FERRO-ALLOY PERCENT C = ((1/KT)(PERCENT CR)**3(P))**1/4 WHERE KT IS THE EQUILIBRIUM CONSTANT FOR THE CHROMIUM OXIDE-CARBON REACTION AT TEMPERATURE T, SAID TEMPERATURE T BEING THE TEMPERATURE OF SAID MOLTEN FERRO-ALLOY DURING SAID PROCESS AND BEING WITHIN SAID ADJUSTED RANGE AND WHERE PERCENT CR IS APPROXIMATELY THE VALUE OF SAID CHROMIUM PRESENT IN SAID FERRO-ALLOY DURING SAID PROCESS AND WHERE P IS THE PRESSURE SURROUNDING SAID MOLTEN FERRO-ALLOY DURING SAID PROCESS. 